This invention relates to a luminaire and, more particularly, to a luminaire of the type in which there is provided about a light source a reflector of substantially transparent material having a prismatic outer surface comprising many double-reflecting prisms with reflecting surfaces oriented to effect total internal reflection of light from the light source. The invention also relates to means for and a method for controlling the amount of light from said source that is allowed to pass through said prismatic outer surface and thus produce uplight.
Luminaires that rely upon prismatic reflectors of the above type are conventional and are disclosed in the following exemplary U.S. Pat. Nos. 2,818,500-Franck; 1,259,493-Dorey; and 1,758,977-Rolph. These prismatic reflectors rely upon the principle of total internal reflection of visible light at a glass-to-air interface when the light is incident upon that surface at an angle greater than the critical angle of incidence, which for a glass-to-air interface is typically about 42.degree.. Typically, each prism has two reflecting surfaces, each of which reflects light onto the other, and each being oriented to provide an incidence angle of greater than the critical angle with respect to light incident thereon. The reflecting surfaces of each of these prisms meet at an apex, and the adjacent reflecting surfaces of juxtaposed prisms meet in a valley between the prisms at the nadir of the valley.
A disadvantage of this type of reflector is that if a non-uniform film is allowed to deposit on the prismatic surface, the desired reflecting properties of the surface may be significantly degraded, especially if the film is partially or fully light-absorbing. If the deposit is not uniform, it effectively changes the incidence angle of light rays striking the relevant interface of the prism with the adjacent air and may let light through this first interface or may change the angle of reflection so much at the first interface that the incidence angle at the second interface is less than the required critical angle, thereby letting light pass through at the second interface.
Another characteristic of prismatic reflectors of the above type is that a substantial amount of light will pass through the reflector even though the reflecting surfaces of the prisms are clean. One reason for this is that the molds typically used for making the prismatic reflectors are not precise enough to achieve mathematical precision of the reflecting surfaces all the way to the apices of the prisms and to the nadir of the valleys between them; and, consequently, light leakage will occur in these regions. Additional light leakage can occur at points of defects in the prism surfaces, which points may be present as a result of mold imperfections or even designed-in defects. Altogether, this light leakage can typically amount to about 20% of the lumen output of the luminaire. In certain luminaire applications, specifically, those with bare reflectors, this light leakage has been advantageously employed to provide uplight, so as to reduce ceiling contrast and to illuminate overhead structures.
The amount of uplighting required from a luminaire will often differ from one lighting application to another. It is therefore desirable that some means be available to the luminaire designer to enable him to change the amount of uplighting, depending upon the particular application. One way that this can be achieved is by intentionally molding defects into the prisms (as is disclosed, for example, in U.S. Pat. No. 4,118,763 - Osteen, assigned to the assignee of the present invention). Corresponding irregularities can be built into the molds used for making the reflector, and these will consistently produce the desired defects in the molded glass or plastic prism structure. A disadvantage of relying upon this approach alone is that once the mold is finished, the amount of uplighting available from the reflector to be molded is fixed, thus restricting the number of applications with different uplighting requirements for which these molds can be used.
Another possible way of increasing uplighting is to add blemishes to the already molded reflector in its active prism area by some approach such as sand blasting, scratching, or etching. In each case this involves a secondary operation that is relatively difficult to adequately control and may result in a rather unsightly finished reflector.